Metal oxides including transition metal oxides, as the main active phase for most catalytic reactions, generally can provide the required high specific surface area for catalytic reactions, wherein the d and f electron bonding energy level of transition metals are especially suitable for the chemical adsorption and activation of the reactant molecules. Additionally, metal oxides have wide applications in the material field, which is the main component for a variety of nano-powders, coatings and inorganic pigments.
The common methods for obtaining metal oxides are as follows: precipitation, hydrothermal synthesis, inorganic metal salt decomposition (e.g., most commonly, the decomposition of nitrates), organic metal salt decomposition, and explosion methods.
Precipitation (co-precipitation) method, for its operability and controllable preparation conditions, usually is the most important preparation method for obtaining metal oxides in industry and laboratories. Precipitation (co-precipitation) method is to allow the soluble salts of one or several selected metal oxides react with the aqueous solution of alkaline substances under a relative low temperature (<100° C.), an atmospheric pressure and a specific pH, to obtain a precipitate of hydrates of metal oxides, and after further treatment, metal oxide catalyst and other metal oxides materials can be obtained. Metal oxides with a desired specific surface area and a grain size can be obtained by controlling conditions of co-precipitation method. Hence, the method is widely used in preparation of a variety of industrial catalysts and metal oxides materials.
Hydrothermal synthesis method is a commonly used preparation method to prepare mixed metal oxides, molecular sieve and nano-powder at relatively high temperature (>100° C.), pressurized and a specific pH. Depending on various products, different raw materials and operation conditions are selected. It generally needs to be carried out in sealed pressurized container under strictly controlled operation conditions.
Metal salt decomposition method is to directly heat inorganic metal salts or organic metal salts in air or oxygen, and make them decomposed to obtain metal oxides.
Presently, metal oxides catalysts used in chemical industry are mainly prepared with conventional precipitation (co-precipitation) technology, the examples of these metal oxides catalysts are as follows: the high temperature (medium temperature) and low temperature shift iron catalysts used in the industrial process of synthetic ammonia; various copper base catalysts in the industrial process of producing methanol and dimethyl ether (DME); catalysts for synthesizing low carbon alcohols and FT synthesis catalysts for converting syngas to hydrocarbons. Additionally, precipitation (co-precipitation) technology is also widely used in preparing metal oxides pigments, materials, superfine particles and nano-powder processes.
The present invention mainly relates to a production method and application associated with the preparation of FT synthesis catalyst.
Most of the preparation methods for FT synthesis catalysts used in coal-to-liquids (CTL) and gas-to-liquids (GTL) industrial production are the aforementioned precipitation (co-precipitation) method. Currently, a large number of patents and literatures have focused on this method.
U.S. Pat. Nos. 4,617,288 and 4,686,313 of Mobil (USA) disclosed a continuous co-precipitation method to prepare Fe—Cu—K FTS catalyst with low nitrogen content by using iron nitrates, which is suitable for industrially continuous production. In U.S. Pat. No. 4,994,428, a method for preparing co-precipitated Fe—Cu—K catalyst and synthesizing hydrocarbon was disclosed, and the obtained hydrocarbon contains more than 90% C5+. U.S. Pat. No. 6,844,370 of Sasol Technology Ltd. also disclosed a catalyst preparation method for a precipitated Fe—Cu—K FTS catalyst without binders, which is suitable for hydrocarbon synthesis in high temperature fluidized bed. U.S. Pat. No. 5,504,118 and CN1113905 of Rentech disclosed a FTS iron base catalyst preparation method used in slurry bed reactor, wherein the nitrates were obtained by dissolving iron and copper with nitric acid, followed by co-precipitation with ammonia, filtrated and washed, then adding potassium carbonate solution, preparing into slurry, and finally granulating by spray-drying. U.S. Pat. No. 6,787,577 of Chevron (USA) disclosed a method for catalyst preparation using co-precipitation of organosilicon and iron salt and its application in FTS. U.S. Pat. No. 5,100,556 of Exxon (USA) disclosed a catalyst preparation method of precipitated Fe—Zn—Cu—K catalyst and its application in FTS, wherein the Fe/Zn mixed nitrates were co-precipitated with ammonia. This method improved the activity and stability of the catalyst and facilitated the production of α-olefins. Patent Nos. CN1600421, CN1600420, CN1817451 and CN1695804 of Shanghai Yankuang Energy R&D Co. Ltd. also mentioned an iron catalyst preparation method by co-precipitation of iron nitrates.
Multiple patents such as CN1463794, CN1597105, CN1395993, CN1583259, CN1562471, CN1463793, CN1562476 and CN1395992 of Synfuels China Co. Ltd. disclosed a series of catalysts preparation method including co-precipitation of iron salts for preparation of Fe/Cu series catalysts and Fe/Mn series catalysts and their applications in FTS. Also, Sasol (South Africa) in its Chinese patent CN1451035 referred to a method of iron nitrate precipitation for FTS catalyst preparation.
The disadvantages of the current precipitation method lies in the following: the generation of a large amount of waste liquid and ammonia nitrate solid; the huge quantity of waste liquid to be treated, especially the explosive properties of the obtained solid ammonia nitrate which brings about some degree of danger, will results in serious environmental issues. In this method, the cost for handling the waste liquid and waste is relatively high, which makes the environment protection issue become an obstacle to the development of catalyst production.
The molecular sieve type catalysts and some oxides carriers used in the petrochemical industry are prepared with the widely adopted hydrothermal reaction method, and this method is also adopted by more metal oxides powders, such as those described in U.S. Pat. No. 5,913,175, CN1558426 and so on. The hydrothermal method is generally carried out in a sealed pressurized container, and the operation conditions are strictly controlled, otherwise, the products with the expected properties cannot be obtained.
Additionally, it is generally difficult to obtain metal oxides materials with uniform particles using thermal hydrolysis method of inorganic metal salts or organic metal salts solution; in addition, it is very difficult to uniformly disperse the additives.
The present invention discloses a green method for preparing hydrates of metal oxides, metal oxides materials and catalysts, which integrates the atmospheric precipitation (co-precipitation) method and hydrothermal reaction method. The method overcomes the drawbacks of the regular hydrothermal reaction method, namely the severe reaction conditions and the requirement of performing the reaction in a sealed container; the method also overcomes the problems of the large consumption of water due to the introduction of one or more chemical reagents and the generation of a large amount of waste water and gas in the atmospheric co-precipitation method. The whole process completely achieves continuous production, enclosed circulation and zero release. The method of the present invention can be widely used in preparation of hydrates of metal oxides, metal oxides materials and catalysts which are used in a variety of chemical industry processes, specifically suitable for preparation of FTS catalyst for converting syngas to hydrocarbons, thus accomplishing the present invention.